Background - the eye and the retina

The most sensitive part of the retina is the fovea. Further away from
the fovea neurons have larger receptive fields a poorer resolution.

Th e eye can move along 3 axes
of movement.

Types of eye movements

Gaze shifting (orienting) mechanisms - Voluntary

Saccades - fast, ballistic, movements

Smooth pursuit - for tracking a moving object

Vergence - to adjust for the distance of the object to be
viewed

Saccades - jerky movements for scanning the scene

Smooth pursuit - following movements

Vergence - adjust to near/far objects

Stabilising mechanisms - Involuntary

Vestibulo-ocular reflex (VOR)- for correcting for head
motion

Optokinetic reflex (OKR) - made in correction to the visual scene moving

Spinning people round on a chair causes them to
make eye movements to compensate- VOROKR can be seen when people
look out of train windows - it manifests itself as nystagmus movie Some people
experience nystagmus spontaneously, in this case it is considered a visual
dysfunction http://www.nystagmus.co.uk/facts/the-basics

Further miniature eye movements

Microsaccades

Drifts and tremor

These small eye movements happen when we are fixing
our eyes (fixating) on an object. You can read all about them here.

Measuring eye movements

There have been various ways developed, but most commonly now
an eye-tracker is used.

This involves a small camera focused on the eye and software
that tracks where the pupil moves.

By calibrating the pupil position against fixed points on the
screen one can work out where the participant is looking.

As well as
the examples shown above, remote eye tracking (with no restriction on the head
is now possible using two remote image sensors to capture images of the eyes
and the reflection patterns. Using this kind of technology we
can build a picture over time of where a participant was looking.

We
can measure:

trajectory

frequency of fixation

dwell time

frequency of fixation

pre-define regions of interest

How to represent and analyse eye movement data

Scanpaths and fixations

Can simply trace the eye movements, or mark each fixation
either with the time spent ateach point or with a circle that represents
dwell time by its size.

Top image taken from Yarbus (1969), Chapter 7 hereBottom
image taken from eye tracker company Tobii website here:
http://www.tobii.com

Heatmaps and regions of interest

In this heatmap taken from Mosimann et al. (2004), where
the warmer colours represent more fixations - in this study they were
looking at differences in attention in Alzheimer's sufferers.

In this study by Norbury et al. (2009) they chose regions of
interest within movie clips of social interactions and compared the amount of
fixations between different developmental groups within these regions e.g. eye,
mouth, body.

Why do we need eye movements?

So we don't have to walk around like pigeons!!!They don't have
eye movements and bob their heads to compensate for their own motion and keep
the image stable on their eye.We use eye VOR and OKR to ensure
stabilisation of the visual scene on the back of the eye (the
retina).

To avoid fading of the visual scene, if a retinal image doesn't
change neurons stop responding - eye movements act as a 'refresh'. This fading
is called Troxler fading.

In order to focus the most important part of the scene on the
fovea, the part of the retina with highest resolution. The image on the
left illustrates equivalentsize of letters according to our ability to see
them if we are looking in the middle of theimage

Why don't we notice eye movements?

In order to build up a detailed representation of the scene we are
constantly moving our eyes, yet we do not notice this motion.

“Saccadic suppression” (Zuber & Stark, 1966) means that
stimuli are more difficult to detect during the movement of the eyes.

In particular it appears that the motion of the whole visual scene is
ignored.

If a change is introduced in the scene whilst we make an eye movement
it is difficult to notice.

This is still a much investigated area – the question of
perceptual stability across consecutive saccades

The brain needs to know which part of the motion on the retina
is caused by eye movements and which part is caused by something moving in the
scene.

This can be done by cancelling out the signal using an
“efference copy” from eye muscles

Also, cues from the scene, such as whole image motion can be
cancelled out.

Some illusions are caused by our inability to cancel out the
motion of the eyes.

The importance of the active vision approach

A lot of research on vision perception ignores the role of eye
movement.

Often the eye as just receiving information from the outside world
that happens to be there.

The active approach emphasis the dynamic process of the sampling of
the visual scenes with reference to the role of the observer in guiding eye
movements to relevant parts of the scenes.

Active vision emphasises the role of eye movements linked with
attention in real-life tasks. Eye tracking studies such as Land et al. (1999)
have measured how eye movements are made on to the next task, before the current one is completed.

Eye movements during reading

Reading is complex cognitive process.

Many models exists to try and describe the ways in which we
extract meaning from written words.

Eye movements are part of the evidence that can be used to test
these models.

The brain needs to know which part of the motion on the retina
is caused by eye movements and which part is caused by something moving in the
scene.

They can also be used to manipulate the type of information
present at fixation at any given time - called a gaze contingent
paradigm

Aoccdrnig to a
reresearchs at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the
ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat
ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll
raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey
lteter by istlef, but the wrod as a wlohe. It has been
shown that we do not fixate on each letter. Typically we move about 8
characters with each saccade. Many short words we do not even fixate on at all.
The example above illustrates that reading does not rely on fixating each
letter in order as we are still bale to make sense of the paragraph,

Gaze contingent paradigmThis involves
displaying the text according to where the eye is looking e.g.:Normal
… move smoothly across the text… Shown . …xxxxxxmoothly
axxxxxxxxxx….. Eye position
............*..............................This can be used to see how many
letters around fixation we use for reading.

The link between eye movements and attention

We are not able to process all visual information at once, we
are only aware of a small part at any given time.

Usually where we are looking is where we are paying attention
to (although not always).

The question is what makes us select something and how do we
achieve this selection. How are eye movements and attention linked? This will
be explored in Lecture 8.

Summary

There are eye stabilising and orienting mechanisms.

Complex processes mean we are constantly able to compensate for our
own motion as well as motion in the visual scene and the movement of our own
eyes.

Eye movements are closely linked with attentional mechanisms and are
the key to seeing vision as an active process.

This involves closely coupled interaction of visual and motor
processes at all levels.